Chlorine, sulphur and nitrogen in wood pellets

Why these elements matter

Chlorine (Cl), sulphur (S) and nitrogen (N) have little effect on the amount of energy in the fuel. They do, however, affect what happens in the boiler and in the chimney: corrosion of metal components and emissions of pollutants to the air. European studies of wood fuels treat them as parameters relevant to corrosion^[46].

Nitrogen content is determined according to ISO 16948, and sulphur and chlorine according to ISO 16994, whose current 2026 edition replaced the withdrawn 2016 version^[22],[23]. Results are given as a percentage by mass on a dry basis^[1]. A useful conversion: 0.01% equals 100 mg/kg.

Chlorine, sulphur and corrosion

During combustion, chlorine passes into the flue gas mainly as hydrogen chloride and volatile chlorides, especially potassium chloride. These compounds deposit on the cooler surfaces of the heat exchanger and accelerate corrosion (Deterioration of metal through chemical reactions with gases and deposits — in a boiler, mainly on the heat exchanger surfaces and in the flue.)Deterioration of metal through chemical reactions with gases and deposits — in a boiler, mainly on the heat exchanger surfaces and in the flue. of the steel. Sulphur forms sulphur oxides, which together with water vapour can produce acidic condensate in the cooler parts of the system, such as the flue.

The effects build up slowly: a single delivery of poorer fuel will not destroy a boiler, but fuel with elevated chlorine content burned season after season shortens the life of the heat exchanger and increases deposits. This is one of the reasons why boiler manufacturers specify fuel requirements.

Nitrogen, sulphur and emissions

In small biomass boilers the combustion temperature is too low to oxidise nitrogen from the air in large quantities. The main source of NO_x (Nitrogen oxides (NO and NO₂) — air pollutants formed during combustion; in biomass boilers they come mainly from the nitrogen contained in the fuel.)Nitrogen oxides (NO and NO₂) — air pollutants formed during combustion; in biomass boilers they come mainly from the nitrogen contained in the fuel. is therefore the nitrogen contained in the fuel itself. The more nitrogen in the pellets, the higher the nitrogen oxide emissions tend to be. Sulphur from the fuel enters the flue gas mainly as sulphur dioxide (SO₂), and is partly bound in the ash.

Clean wood contains very little sulphur, so SO₂ emissions from good pellets are low. For the same reason the sulphur limits are equally low for all ENplus classes^[1].

Threshold values

In milligrams: the class A1 limit is 200 mg/kg for chlorine, 400 mg/kg for sulphur and 3000 mg/kg for nitrogen. The only difference between ENplus and the ISO draft standard concerns sulphur in class B: ENplus keeps 0.04%, while the ISO draft allows 0.05%^[1],[12]Data require verification: The ISO 17225-2 values were read from the FDIS draft; they need to be confirmed in the published standard.. ENplus also sets limits for heavy metals — we list them on the quality parameters page.

How much clean wood contains

Measurement data show that clean wood contains these elements at levels well below the limits.

In the study by Schmidl and co-workers, which covered log wood and pellet samples from 14 European countries, only two samples — one of log wood and one of pellets — contained more than 0.01% chlorine, and none exceeded 0.02%. The authors noted that literature values were considerably higher than their measurements — in the case of chlorine by a factor of about ten — partly because the limits of quantification of the methods had been reported as results^[46]. According to VTT, wood usually contains less than 0.05% sulphur — 0.0090–0.0116% in stemwood in this dataset — while tree bark contains about 0.034%, clearly more than stemwood itself^[44]. Of the species included in that compilation, grey alder had the highest nitrogen content (0.19%)^[44].

Where elevated values come from

Since clean wood contains little of these elements, a result close to or above the limit needs an explanation. The most common causes are:

• chemically treated wood — wood-based panels with adhesives (urea-formaldehyde adhesives contain nitrogen), and wood that has been varnished, painted, impregnated or coated with plastics (a source of chlorine);
• a large share of bark, leaves and needles — parts of the tree richer in mineral matter, nitrogen and sulphur than stemwood;
• contamination from the surroundings — for example raw material stored where it came into contact with road salt, fertilisers or waste.

ENplus prohibits demolition wood and chemically treated wood in all classes^[1]. ISO 17225-2 is more lenient here: in class B it allows chemically treated by-products of the wood-processing industry and chemically untreated used wood, provided that the content of heavy metals and halogenated organic compounds does not exceed the values typical of virgin wood^[11].